Necessary party to suit

My mother has filed the suit of specific performance. The sale agreement was in my mother and father name jointly. My father died before filling suit. The suit had been solely filed by my mother. Me and my sister are not party to the suit of specific performance which my mother filled and also do not want to be party in future. Can my mother solely get decree of suit of specific performance of agreement. Is me and my sister necessary party to suit. It is also mentioned that the defedant has not raised any objection of non joinder of necessary party in written statement or separately till now for me and my sister Like if in future defedant raise this objection can we be added that time. Will my suit be dissimissed even if no objection of non joinder of necssary party by defdant or no issue is framed for the same